User-friendly input macros.

Add `scan!`, `scanln!`, etc. macros which mirror `print!`, `println!`, etc. for doing straightforward input from stdin, a file, or other Reader.

I really miss having something like this. I think it would be really good for increasing uptake of Rust to have a good story here.

I was hacking on a prototype implementation over the weekend. It only supports `{}` holes (although some of the infrastructure for fixed widths is there) and doesn't work yet, but its getting there. I'd like to land something like that and then extend the mini-language gradually, letting the design evolve.